The present invention relates to an imaging device and, more particularly, to an improvement in an output circuit incorporated in a solid-state image pick-up device such as a charge-coupled device (CCD).
The CCD is widely used as a solid-state imaging device to convert light into electric signals. The CCD is therefore incorporated into video cameras, facsimiles and so on. In order to output electric signals, the CCD has an output amplifier which is disclosed in Japanese Patent Laid-Open Application No. 4-298176 for example. The output amplifier disclosed therein employs transistors, each of which is of a surface type. For this reason, there occurs a problem that the noise of the output amplifier is large.
To solve such problem, a new output amplifier has been invented by a co-pending patent application Ser. No. 568,930/08 which was filed on Dec. 7, 1995.
Referring FIG. 14, there is shown a high-vision CCD employing such new output amplifier. This CCD has photodiodes 19, vertical CCD registers 20, and a horizontal CCD register 21. An output amplifier 22 is connected to the horizontal CCD register 21. A signal photoelectric-converted by the photodiode 19 is transferred via the respective vertical CCD register 20 into the horizontal CCD register 21 and is amplified by the output amplifier 22 to produce an output signal. The vertical and the horizontal registers are driven a driving signal respectively.
The output amplifier 22, as shown in FIG. 15, employs three-stage source-follower amplifier circuit on chip. The output amplifier 22 has three driver transistors 23, three load transistors 24, a V.sub.DD terminal 2, a GND terminal 6, and a V.sub.out terminal 12. An input terminal of the output amplifier 22 is connected to a floating connection or floating junction (FJ) capacitor 25 and a RESET transistor 26 having a RESET drain terminal 27. The driver transistor of the first stage is formed by surface type in order to hold linearity thereof. The other transistors are formed by buried or embedded type to reduce noise.
Signals applied from the horizontal CCD register 21 is supplied to the connection node of the FJ capacitor 25 and the RESET transistor 26. After the signals is amplifiered by the output amplifier 22, the RESET transistor 26 is active and the level of the connection node of the FJ capacitor 25 and the RESET transistor 26 is equalized with the level applied to the RESET drain terminal 27.
At present, CCD cameras with a multifunction have been developed so that they correspond to multimedia techniques. There therefore is a demand for an application such as photographing a dynamic image or a moving picture and a still image or a still picture with the same camera.
When a still image is photographed or picked out, the CCD camera does not need to drive with a high speed compared with that when a dynamic image is photographed. That is, the vertical and the horizontal registers do not need to drive by the driving signal with high frequency in a still image photographing mode. Therefore, the drive frequency in a still image photographing mode can become low compared with a dynamic image photographing mode. However, the characteristic of the conventional output amplifier has been arranged with the dynamic image photographing mode. Therefore, a conventional semiconductor device incurs waste of gain bandwidth product and consumption of power when it drives in a still image photographing mode.